Jan Kuciak’s murder triggered protests that led to the resignation of Pellegrini’s predecessor, Robert Fico.
Photograph: Joe Klamar/AFP/Getty Images

The prime minister of Slovakia has hit out at people seeking to portray his country as a mafia state following the murder of a reporter investigating government corruption, but conceded that the “professional” killing was proving difficult to solve.

Peter Pellegrini, who replaced Robert Fico after he was forced to quit in the wake of the murders of the journalist Ján Kuciak, 27, and his fiancee, Martina Kušnírová, said unprecedented numbers of police were working on the case.

The authorities believe Kuciak was killed in relation to his work. He had recently been examining claims that a senior aide to Fico had business links with a man suspected of tiesto the Italian mafia.

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In an interview with the Guardian and five other European newspapers during a visit to Brussels, Pellegrini said the Slovakian police were being assisted by the FBI, the Metropolitan police and Italian and Dutch forces, but he appeared to suggest the investigation was struggling because of the delay in finding the bodies.

“I am very sorry for the fact that the murders are running four days ahead of us. The problem is that the murder did occur on Wednesday, but the people were reported missing on the Sunday. This is far too much in case of a murder,” he said.

“It seems it was ... very professional. That is why it may be very difficult to find. But for my government, for myself, for our citizens it is very important to find the perpetrator as quickly as possible.”

Pellegrini was Fico’s deputy for the last two years, and while the protests that forced the former prime minister from office have been put on hold, there are concerns among those campaigning for a cleanup of government corruption that little will change.

Kuciak’s final piece of work, completed by a consortium of journalists, investigated why Fico had hired Mária Trošková, a then 27-year-old former model and Miss Universe contestant, as one of his assistants despite what it described as her relative lack of political experience.

Trošková had been a business partner of Antonino Vadala, 42, an Italian living in Slovakia with alleged close ties to Italy’s ’Ndrangheta mafia group.

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Pellegrini, who described Kuciak’s murder as an attack on the freedom of the press, said: “My first task is to rebuild the image of Slovakia and I will do the utmost to make sure that people will not portray Slovakia with an unfair image.

“Slovakia has recently been placed as 17th in the world in terms of freedom of the press. Slovakia continues to score highly in terms of security and crime rate. I really strongly refute the image of Slovakia as a country being run by the mafia, which is an image being spread by several individuals or groups of people.”

Pellegrini was in Brussels to meet the president of the European commission, Jean-Claude Juncker, and the European council president, Donald Tusk. He said his government would be ardently pro-European, but that it would continue to reject the EU’s quota system for the dispersal of refugees.

Pellegrini suggested Juncker and the commission understood the system of mandatory relocation would not work. “We are offering some other measures”, he said. “Some solidarity measures. We are prepared to increase our contribution, help other countries with technical support, financially, or something like that, but please, it is very difficult in our areas to talk about mandatory quotas.”